A laser emits light (electromagnetic radiation) through a process of optical amplification based on the stimulated emission of photons. Light emitted from a laser is notable for its high degree of spatial and temporal coherence. Spatial coherence typically is expressed through an output being a narrow beam which is diffraction-limited, often a so-called “pencil beam.” Laser beams can be focused to very tiny spots, achieving a very high irradiance. Alternatively, laser beams may be launched into a beam of very low divergence in order to concentrate power at a large distance.
Temporal (or longitudinal) coherence implies a polarized wave whose phase is correlated over a relatively large distance (the coherence length) along the beam. A beam produced by a thermal or other incoherent light source has an instantaneous amplitude and phase which vary randomly with respect to time and position, and thus a very short coherence length. The degree to which a laser is temporally coherent can depend on the spectral properties of the laser emission.
A laser can emit light at one or more wavelengths defined by the longitudinal modes of the laser cavity. The spacing of these modes can vary inversely with cavity length. A laser that emits light predominately in one of these cavity modes can be said to be a “single mode” or “single wavelength” laser. The degree to which the single mode laser is operating in single mode is defined by the side mode suppression ratio (SMSR), which defines the ratio of the power in the predominate mode to the power in the other modes. Typical applications would require SMSR greater than 30 decibels (dB), and some applications require SMSR greater than 50 dB.